Disk drives typically arrange data as blocks or sectors within concentric data tracks defined on a storage surface of a rotating medium. In magnetic disk drives, the storage surface includes a magnetic material which serves as the storage medium. In Winchester or flying-head disk drives, known as hard disk drives, a magnetic read-write element or elements is carried upon a slider which flies in very close proximity to the data storage surface upon an air bearing. Data is written to sectors by the write element, and data is read from the sectors by the read element.
As disk drive storage capacities have continued to escalate upwardly, new and more advanced techniques have been employed to effectively increase the amount of data per unit areal density of the storage medium. One technique has been to employ a synchronously sampled data detection channel in place of an older peak detection channel. In a peak detection channel magnetic flux transitions must be spaced sufficiently apart to avoid pulse crowding. Pulse crowding lead to two drawbacks for peak detection channels. The first is that the amplitudes of the pulses are altered. The second is that the positioning of the peaks of the pulses are shifted, leading to peak shift, a form of intersymbol interference. The synchronously sampled data detection channel is more powerful than peak detection techniques in that coded information bits, represented as flux transitions, may be placed more closely together and still resolved as information than was possible with peak detection techniques. One example of a synchronously sampled data detection channel is provided by commonly assigned U.S. Pat. No. 5,341,249 to Abbott et al., entitled: "Disk Drive Using PRML Class IV Sampling Data Detection with Digital Adaptive Equalization", the disclosure thereof being incorporated herein by reference.
As the magnetic disk drive read channel detection schemes have become more powerful, the amount of digital processing of the channel output has increased. This increase in digital processing has resulted in increased read latency in the detector. For a hard disk drive with a relatively small sector size (generally 512 user bytes, but sometimes 256 user bytes or less) and possibly including ID fields and split sectors, the larger read latency is increasing the amount of wasted disk space ("pad") which is required between sectors. Unless special measures are taken, it is necessary to finish processing of a previous sector before beginning processing of a next sector. This problem is exemplified in the uppermost graph of FIG. 1 which illustrates (in a linearized depiction) two disk sectors S1 and S2. Two pad fields separate sectors S1 and S2. The first pad field P1 is a digital process latency pad field (which is wasted disk storage space required to enable digital processes to complete the processing of the S1 information before beginning processing of the S2 information). The second pad field STB represents a small spindle speed tolerance buffer pad which is required to accommodate disk rotational tolerances within the disk drive and from drive to drive within a product family.
As an example, a synchronously sampled data detection channel will typically include a Viterbi decoder including a memory path, and a channel decoder. In addition, an interface to a disk sequencer or controller will include a parallel-to-serial conversion process. As more complex partial-response polynomials and advanced coded channel schemes with long block lengths become common (such as trellis coded partial-response), significant increases in Viterbi decoding and channel decoding may result. Typical delays for Viterbi decoding and channel decoding are 10-50 bits and 8-40 bits, respectively.
With contemporary PRML read channels, a decoding delay of six to ten bytes is typical and will likely increase as detectors become more complex. Depending on the overall drive architecture (e.g. split sectors and/or ID fields), this delay can increase disk overhead by as much as two to four percent (2-4%) of a sector. Thus, in the example given in FIG. 1 the pad field P1 would have a length of six to ten bytes in order to accommodate digital processing latency. The read gate line RD GATE will be enabled at the beginning of S1 and S2, and there will be some process latency before the channel begins to put out digital data from the digital processes including the Viterbi detector and channel decoder, etc. This delay is marked on the second graph of FIG. 1 as SD (for start data). Digital data continues flowing out of the channel pipeline until all of the user data read from sector S1 has been processed and flushed out, as marked by the falling edge of RD GATE (also identified as ED for end of data). RD GATE remains unasserted throughout the speed tolerance buffer STB and rises to an asserted level at the beginning of the second sector S2. The third graph illustrates a time window when channel data is valid and this time window co-extends with the interval between start data SD and end of data ED in the second graph of FIG. 1. The hatched region of the read data valid graph represents the digital process latency interval after the analog channel has completed delivering the information recorded in sector S1 and is coextensive in time with the length of the pad field P1.
In the field of magnetic recording employing multiple heads for scanning a magnetic recording medium in helical fashion to remove data block skew between heads. One example of this prior art is provided by U.S. Pat. No. 4,775,899 to Pasdera et al., entitled: "Apparatus for Deskewing Successively Occurring Blocks of Data". Nevertheless, it has been recognized by technologists in the field that at least one drawback for trellis-coded partial response channels has been the relatively long digital processes had negative implications for throughput rate, decoding delays and track format overhead, see Christiansen, Fredrickson, Karabed, Rae, Shih, Siegel and Thapar, "Design and Performance of a VLSI 12 MB/s Trellis-Coded Partial Response Channel", The Magnetic Recording Conference, San Diego, Calif., Aug. 15-17, 1994, paper F-6.
Accordingly, a hitherto unsolved need remains for a practical method for overlapping block read events in a disk drive in a manner overcoming limitations and drawbacks of the prior art.